This invention relates generally to methods for conveniently and efficiently making a projection screen and more particularly to projection screens constructed according to such methods.
Recycled glass is generally glass that has been previously used for a particular purpose and then discarded. Examples include automobile windshields, beverage bottles and windows that have been broken into small pieces. It is known to form recycled glass into low quality microspheres. Recycled glass microspheres are available from Flex O Lite of Muscatine, Iowa, Paris, Texas and St. Thomas, Canada; and Potters of Valley Forge, Pa. Recycled soda lime glass has a refractive index of about 1.5.
FIG. 4 illustrates an example of glass microspheres constructed from recycled glass. Recycled glass microspheres are considered to be lower quality microspheres than similar microspheres generated from substantially pure raw materials. Note the glass has several non-spherical defects. Glass microspheres constructed from recycled glass are typically used as inexpensive filler materials to modify the flow characteristic of a material, or for peening processes, or as an reflective component in a reflective article such as a road marking product. When used in these processes, a large number of defects (e.g., cullet, opaque particles and bubbles) may be tolerated. Glass microspheres constructed from recycled glass are available in a wide range of sizes. The standard deviation of the size of such recycled glass spheres is typically quite large.
A rear projection screen is a sheet-like optical device having a relatively thin viewing layer that is placed at an image surface of an optical projection apparatus. Such a screen makes visible a real image focused by a projection apparatus onto the image surface. The viewing layer is typically planar corresponding to the image surfaces produced by a projection apparatus. Other shapes are possible if the image surface of the projection apparatus is not planar. The screen is intended to act as a filter to attenuate, block, or diffuse light which is not part of the projected image, and to transmit from its rear side to its front side that light which is part of the projected image. In this way it enables the viewer to see the projected image when looking at the front side of the screen.
Front projection systems are also known in the art. They comprise a projector designed to project an image on a surface (e.g., the wall of a conference room or a screen). Overhead projectors are an example of a front projection system. Front projection systems have less capacity to absorb ambient light than rear projection systems.
A well-known type of rear projection screen is a thin, light diffusing layer such as a frosted or translucent glass surface, which may be produced by etching, sandblasting, or otherwise roughening a smooth glass surface. The translucent surface limits the visibility of objects behind the screen. The screen must, however, be sufficiently light transmissive to allow the projected image, which is focused precisely on the translucent surface, to be viewed from the front side of the screen. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent suffer, however, from a tendency to strongly reflect ambient light incident on the front side, thereby causing fading, or washout, of the projected image. This problem is particularly severe if the background or ambient light is bright.
An approach to reducing the effects of ambient light while still maintaining an acceptable level of projected image light is to attach an array of closely packed glass microspheres (i.e., beads) to a substrate by an opaque polymeric binder. The glass microspheres and substrate are both light transmissible (e.g., transparent). The glass microspheres act as lenses to collect projected light from the rear of the screen and focus it to relatively small spots, near the surfaces of the microspheres. The foci are approximately in the areas where the microspheres contact the front support layer.
Because the transparent microspheres contact the front of the substrate, they exclude most of the opaque binder material from the space between the microspheres and their contact areas on the substrate. This forms an optical aperture between each microsphere and the substrate. The area surrounding each optical aperture is opaque, and preferably black, due to the opaque binder material in the microsphere interstices. As a result, ambient light incident in these areas is absorbed. Thus the front side of the screen appears black, except for the light transmitted through the microspheres.
U.S. Pat. No. 2,378,252 (Staehle) describes projection screen displays, such as rear projection screen displays, based on glass microspheres embedded in an opaque matrix. These types of screens have been growing in popularity for various uses, such as in large format televisions. Rear projection screens with glass beads are also disclosed in U.S. Pat. Nos. 5,563,738 and 5,781,344. Additional beaded screens and methods of making such screens are disclosed in commonly assigned patent application PCT WO 99/50710.
The appearance of such screens is highly sensitive to the quality and placement of the glass microspheres used. Excessive amounts of microspheres that are of incorrect size, are not spherical, or are broken, nicked, scratched, or otherwise defective can create a variety of visible defects, variously called graininess, scintillation, sparkles, speckle, punch through, or simply spots. For example, U.S. Pat. No. 5,563,738 (Vance) states that it is xe2x80x9cnecessary to eliminate out-of-round, wrong-sized, and discolored microspheresxe2x80x9d in order to obtain a uniform appearance.
PCT WO 98/45753 states that glass bead rear projection screens suffer from limitations, especially when they are used to project high quality images that are to be viewed from short distances. The appearance of such screens is highly sensitive to the quality and placement of the glass beads used. Beads that are of incorrect size, are not spherical, or are broken, nicked, scratched or otherwise defective can create a variety of visible defects, variously called graininess, sparkles, punch through or simply spots. These defects are particularly troubling when the screen is used, for example, as a computer monitor, where the need to see a high level of detail is likely to lead the user to scrutinize the screen closely, from a short distance, for long periods of time.
Sparkle is of particular concern for a rear projection system. Sparkle in a rear projection screen is a random pattern of bright points of light that change with the position of the viewer and results from defective beads.
U.S. Pat. No. 6,172,814 (Wantanabe et al.) discloses a rear projection screen having a plurality of glass minute transparent balls. Wantanabe et al. discloses several embodiments. In those embodiments, glass minute transparent balls having a refractive index of n=1.5, n=1.6, n=1.7, n=1.8, n=1.9 and n=2.1 are disclosed. Wantanabe et al. stated that glass minute transparent balls having a refractive index of n=1.5, n=1.9 and n=2.1 were mass produced, generally available and at low cost. Wantanabe et al. also stated that glass minute transparent balls having refractive indexes of n=1.6, n=1.7 and n=1.8 are comparatively expensive.
U.S. Pat. No. 6,204,971 (Morris et al.) discloses glass microspheres for use in rear projection screen displays and methods of making such microspheres. In a preferred embodiment, Morris et al. tolerates low levels (e.g., less than 15%) of defects. In a preferred embodiment, the defect level was measured by counting the microspheres and the respective defects of a microsphere sample under an optical microscope. The percent defects referred to the total number of microspheres that contained bubbles, possessed odd shapes, were opaque or formed cullet.
A number of processes have been devised for the production of spherical glass bodies in small sizes. These generally involve the special melting of glasses with the desired refractive index, converting the glass into particles and the subsequent free suspension of the particles in a hot zone for a time and at a temperature sufficient to permit each particle to be drawn into a spherical shape by surface tension. Examples of glass bead processing are found in U.S. Pat. Nos. 2,794,301 and 3,150,947.
The present invention provides methods of convenient and efficiently producing glass microspheres for use in projection screens, such as a monolayer rear projection screen. It has been determined that certain defects in a rear projection screen arise, to a large degree, by certain glass bead defects. Elimination or a substantial reduction in those certain defects results in a surprisingly high quality projection screen without the need to engage in additional, expensive processing steps to eliminate the many potential glass bead defects. For example, it has been determined that there is a strong correlation between glass beads with facets and projection screens that exhibit a sparkle defect.
Facets are flat areas that are found in cullet, broken microspheres, cracked microspheres and the interface between duplexes. While sparkle can be caused by almost any bead imperfection, other bead imperfections tend to have a lesser surface area or are rounded and, as a result, tend not to be as objectionable to a viewer of the rear projection screen.
Reprocessing recycled glass beads sufficient to reduce the number of facets reduces the need to engage in additional bead reprocessing specifically designed to reduce other defects such as bubbles, duplexes or opaque particles. Surprisingly, glass microspheres formed from such a process may be used in a projection screen that provides acceptable gain, viewing angle, resolution, transmission, ambient light rejection and contrast performance. For purposes of this application, gain is defined as the ratio of the luminance of the screen on axis to the luminance on axis of a Lambertian source having the same integrated light output. Screens having gains as high as 2.25 are believed possible with the present invention.
The glass microspheres constructed according to the present invention combine a desirable index of refraction (preferably, no greater than about 2.1, more preferably, about 1.4 to about 1.9, and most preferably, about 1.5 to about 1.57) and low levels of defects (e.g., bubbles, visible haziness, frostiness, or opacity, and especially substantially nonspherical shapes) upon being reformed (i.e., xe2x80x9cas producedxe2x80x9d without subsequent sorting to pick out the defects). The bulk, initial recycled glass may be provided with a refractive index within a preselected range.
Preferably, a population of microspheres as produced has less than about 15% defects. More preferably, after sorting, the glass microspheres, as produced, have less than about 5 percent facet defects, and it is believed that the present invention makes it possible to reform recycled glass to provide glass microspheres, as produced, having substantially no cullet.
The process of the present invention is believed to reduce other defects such as bubbles and opaque particles. With the present invention, after sorting, the glass microspheres, as produced, may have less than about 5 percent defects. The terms xe2x80x9cmicrosphere,xe2x80x9d xe2x80x9cbead,xe2x80x9d and xe2x80x9csphericalxe2x80x9d are used herein for rounded, substantially unitary glass elements, which may not be perfect spheres.
Preferably, the glass microspheres are visibly transparent (e.g., they transmit a sufficient amount of light such that they are suitable for use in beaded rear projection screen displays). Microspheres that are suitable for use in displays are preferably less than about 150 xcexcm in diameter.
In one aspect, the present invention comprises method of making a projection screen, preferably a rear projection screen. The method comprises the steps of providing recycled glass, feeding the recycled glass to a reforming furnace flame, reforming the recycled glass into microspheres with the reforming furnace flame, removing glass microspheres outside a preselected range, providing a substrate having an opaque matrix disposed thereon; and applying the glass microspheres to the opaque matrix under conditions effective to produce microspheres in optical contact with the substrate and embedded in the opaque matrix.
The step of providing recycled glass preferably includes the step of providing recycled glass microspheres with a significant number of facet defects. This type of bulk, raw material is believed to be relatively inexpensive and can reduce the overall cost of the projection screen constructed according to the present invention while maintaining acceptable quality performance levels.
Preferably, the step of reforming the recycled glass into microspheres with the reforming furnace flame, further comprises the step of adjusting the reforming furnace flame. More preferably, it includes the step of adjusting the flame stoichiometry sufficient to reform at least some of the recycled glass with facets into microspheres without substantial reboiling or remelting of the recycled glass.
Additional optional steps may be included in the invention including the steps of choosing an acceptable size range for the reformed, recycled glass microspheres, cooling the recycled glass after the recycled glass is formed into glass microspheres, and sorting the cooled microspheres to remove microspheres outside the acceptable size range.
In another aspect, the present invention comprises a projection screen with recycled glass beads constructed according to a reforming process. The projection screen comprises a monolayer of glass microspheres having an average particle diameter of less than about 150 xcexcm in diameter, wherein the glass microspheres are constructed from recycled glass that has been fed into a reforming furnace flame under conditions sufficient to substantially reduce facets on the recycled glass and to reform the recycled glass into glass microspheres. The screen also includes a substrate, and an opaque matrix positioned such that the microspheres are in optical contact with the substrates and are embedded in the opaque matrix.
The projection screen may be a rear projection screen or a front projection screen with beads having a preselected refractive index and size range.